As a switchgear having a current interrupting function, various types such as a load switchgear, a disconnector, and a circuit breaker, exist depending on use purpose and required function. Most of the above switchgears are constituted such that electrical contacts that can be mechanically opened and closed are placed in a gas atmosphere, and when electrical current passes, the electrical contacts are held in contact for conduction, and when the current is interrupted, the electrical contacts are separated and an arc discharge is produced in the gas atmosphere, and the current is interrupted by extinguishing this arc.
In recent years, for the purpose of obtaining higher current interruption performance, there is proposed a method that obtains higher spraying pressure not only by utilizing mechanical pressure of a piston but also by actively introducing heat energy of the arc into a puffer chamber. For example, there is proposed a method that introduces a movable-side hot gas flow into the puffer chamber through a hole formed in a hollow rod at the initial time of the interruption operation (refer to Japanese Patent Publication No. 07-109744, the entire content of which is incorporated herein by reference).
Further, there is proposed a method that obtains high spraying pressure applied to the arc especially at the time of large current interruption by dividing the puffer chamber into two parts in the axial direction and restricting the volume of the puffer chamber near the arc and reduces driving force for driving a movable contact portion by providing a check valve at the dividing portion of the puffer chamber so as to avoid high pressure from being applied directly to a piston (refer to Japanese Patent Publication No. 07-97466, the entire content of which is incorporated herein by reference).
In a switchgear that has been in widespread use recently, SF6 gas or air is often used as the arc-extinguishing gas. SF6 gas is excellent in arc-extinguishing performance and electrical insulation performance and is widely used in high-voltage switchgears. On the other hand, the air is often used in a compact type switchgear due to low cost, safety, and environmental friendliness.
SF6 gas is very suitable for use especially in a high-voltage switchgear, while it is known that SF6 gas has a high global warming effect and a reduction in use of SF6 gas is demanded in recent years. In general, the magnitude of global warming effect is represented by global warming potential, that is, by a relative value when global warming potential of CO2 gas is set to 1, and it is known that a global warming potential of SF6 gas reaches 23,900. Although the air is excellent in safety and environment conservation property, the arc-extinguishing performance and electrical insulation performance of the air are significantly inferior to those of SF6, gas, so that it is difficult for the air to be widely applied to the high-voltage switchgear, and the use of the air as the arc-extinguishing gas is considered to be limited to a low to middle-voltage switchgear.
Under such a circumstance, a use of CO2 gas as the arc-extinguishing gas in a switchgear is proposed (refer to Uchii, Kawano, Nakamoto, Mizoguchi, “Fundamental Properties of CO2 Gas as an Arc Extinguishing Medium and Thermal Interruption Performance of Full-Scale Circuit Breaker Model”, Transactions B of the Institute of Electrical Engineers of Japan, Vol. 124, No. 3, pp. 469 to 475, 2004, the entire content of which is incorporated herein by reference). CO2 gas has much lower global warming effect than SF6 gas, so that the use of CO2 gas in place of SF6 gas in the switchgear allows an adverse effect on global warming to be significantly reduced. Further, although the arc-extinguishing performance and electrical insulation performance of CO2 gas are inferior to those of SF6 gas, the arc-extinguishing performance of CO2 gas is much superior and insulation performance is equivalent or superior to the air. Thus, by using CO2 gas in place of SF6 gas or air, it is possible to provide a switchgear having satisfactory performance and having environmentally-friendly features in which an adverse effect on global warming is reduced.
In addition to CO2 gas, a use of perfluorocarbon such as CF4 gas, hydrofluorocarbon such as CH2F2 gas (“Global Environmental Load of SF6 and Insulation of SF6 Mixture or Substitute Gas”, Technical report of the Institute of Electrical Engineers of Japan, No. 841, 2001, the entire content of which is incorporated herein by reference), and CF3I gas (Japanese Patent Application Laid-Open Publication No. 2000-164040, the entire content of which is incorporated herein by reference) as the arc-extinguishing gas in a switchgear is proposed from the same standpoint. The gases mentioned above have a smaller adverse effect on global warming and have comparatively higher arc-extinguishing performance and insulation performance than SF6 gas, so that the above gases are considered to be effective for a reduction in environmental load produced by the switchgear.
Further, there is proposed a method in which in the case where the gas containing element C is applied to the switchgear, an appropriate amount of O2 gas and H2 gas is mixed with the element C containing gas so as to suppress the amount of free carbon to be generated at the time of current interruption to thereby prevent electrical quality degradation due to generation of the free carbon (Japanese Patent Application Laid-Open Publication No. 2007-258137, the entire content of which is incorporated herein by reference).
Further, there is proposed a technique in which a hybrid breaker having contactable and separable two pairs of electrodes and one pair of which constituting a vacuum breaker uses mixed gas containing CH4 as insulation gas in one arc-extinguishing chamber (Japanese Patent Application Laid-Open Publication No. 2001-189118, the entire content of which is incorporated herein by reference).
Further, there is proposed a technique in which a circuit breaker containing contactable and separable two pairs of electrodes in individual arc-extinguish chambers uses mixed gas containing CH4 and N2 (Japanese Patent Application Laid-Open Publication No. 2003-348721, the entire content of which is incorporated herein by reference).
As described above, there has been proposed a technique using CO2 gas, perfluorocarbon, hydrofluorocarbon, or CF3I gas as an arc-extinguishing medium to provide a switchgear that reduces an adverse effect on global warming as compared to a conventional switchgear using SF6 gas and has satisfactory performance.
In this case, however, the following four serious problems arise.
The first problem is that: all the abovementioned gases contain element C, so that when any of these gases is applied to the switchgear, free carbon may be generated while the gas is dissociated and recombined by high-temperature are generated at the time of current interruption.
If the carbon generated in association with the current interruption is adhered to the surface of a solid insulator such as an insulation spacer, the electrical insulation performance of the solid insulator may be significantly degraded, which may impair the quality of the switchgear.
Further, in the case where any of the above gases is applied to a puffer-type gas insulated circuit breaker and where the heat energy of the arc is actively utilized as a pressure-increasing means for increasing the pressure of a puffer chamber for the purpose of enhancing the interruption performance, the temperature of the gas inevitably becomes higher than a conventional gas insulated circuit breaker mainly utilizing mechanical compression by means of a piston. When the temperature of the gas is increased, specifically, up to about 3000 K or more, dissociation of gas molecules significantly progresses to make it easy to generate carbon. Therefore, when any of the above gases is applied to the puffer-type gas insulated circuit breaker and when the heat energy of the arc is actively utilized for high puffer chamber pressure, the carbon is increasingly easier to be generated, which may impair the quality of the breaker.
To avoid this, it is necessary to restrict a use of the heat energy of the arc so as to prevent the carbon from being generated, so that the interruption current is restricted to be small or spraying pressure rise required for large current interruption needs to be achieved mainly by mechanical compression, which may increase the size and cost of the switchgear.
The second problem is that: among the gases mentioned above, perfluorocarbon, hydrofluorocarbon, and CF3I gas have a lower global warming potential than SF6 gas but are artificial gases that do not exist in nature, so that when a large volume of these gases is produced for application to the switchgear, greenhouse gases are correspondingly increased on the earth, resulting in an increase in environmental load.
The third problem is that: CF3I gas and most of the gases belonging to perfluorocarbon and hydrofluorocarbon have complicated molecular structure, so that once the molecules are dissociated by the arc, they are likely to be turned into different molecules in the process of recombination. For example, depending on the value of current to be interrupted or gas condition, CF3I gas dissociated by the arc may be recombined into I2, C2F6, and the like. Further, C2F6 gas may be turned into CF4 having a simpler molecular structure. Thus, when any of these gases is applied to the switchgear, composition of the gas is changed every time current is interrupted, which may result in gradual degradation from expected performance.
The fourth problem concerns mixed gas of CO2 and O2 or mixed gas of CO2 and H2. These gases are naturally-derived gases and can be considered to be truly environmentally friendly. Further, as has been proposed in Japanese Patent Application Laid-Open Publication No. 2007-258137, by mixing an appropriate amount of O2 and H2, it is possible to suppress to some extent the first problem, i.e., generation of free carbon after the current interruption even while using CO2.
However, O2 gas is a representative substance that promotes degradation of an organic material or metal and significantly promotes degradation of especially a metal conductive part exposed to high-temperature environment provided by conduction or an organic material such as a rubber packing, an insulator, a lubricating grease, resulting in a reduction in the device lifetime and an increase in the number of times of device maintenances. In particular, an insulation nozzle is exposed to arc having a temperature of up to several tens of thousands of degrees K, so that the damage becomes significant as the concentration of O2 gas having combustion-supporting property increases, which may result in the combustion if the current value or gas pressure is high.
Further, mixed gas of CO2 and H2 has a problem in terms of safety, electrical insulation property, and gas-tightness. H2 gas has extremely high combustion speed among combustible gases, and the explosive range of H2 gas in the air is as extremely wide as 4 to 75%. If H2 gas is leaked at the operating time or gas handling time, explosion is likely to occur. Further, H2 gas has excellent current interruption performance but has extremely low insulation performance (about 10% or less of the current interruption performance of CO2 gas). Thus, when H2 is mixed with CO2 gas, the insulation gap length needs to be increased in order to ensure sufficient insulation performance, resulting in an increase in the device size. Further, the molecular size of H2 gas is small, making it difficult to ensure gas-tightness. As a result, in order to ensure gas-tightness, doubling of a gas packing or the like is required.
Japanese Patent Application Laid-Open Publications Nos. 2001-189118 and 2003-348721 propose a technique that uses mixed gas containing CH4 and N2 in one of two arc-extinguishing chambers. However, an optimum composition of mixed gas has not been established.